by Margaret Dedloff
Technique name: Plaque Assay
Fun Rating: 3/5
Difficulty Rating: 3/5
What is the general purpose?
When growing a virus in culture or collecting infected tissues from mice, it is important to be able to figure out how much virus is in the sample. Because viruses are very small and cannot grow on their own, you need to use specific assays to determine how much virus is present, such as a plaque assay. Plaque assays are used to determine how much virus is present in a sample by counting spots, called plaques. Plaque assays are an easy way to measure the amount of virus quickly and in many samples at once.
Why do we use it?
Viruses are very small and hard to visualize, but many are able to cause cell death. Plaque assays make it possible to visualize cell death caused by viruses and determine how much virus is in the original sample.
How does it work?
Step 1: A single layer of cells, called a monolayer, is grown in a tissue culture plate.
Step 2: Viral samples are added to the monolayers in a dilution series. A dilution series is created by adding the initial virus sample to increasing amounts of diluent, usually saline (salt water). The reason the initial sample is diluted is because if the sample has too much virus in it, the end result will have too many plaques and you will be unable to count them reliably.
Step 3: As the virus enters the cells and kills them, holes appear in the monolayer. These holes are called plaques.
Step 4: The plaques can then be counted and the following mathematical formula is used to determine the amount of virus:
Here is an example calculation based on the final plate in the dilution series from Step 2:
The plaque assay is a very useful tool that scientists can use to calculate the amount of virus in a sample. An important note about that plaque assay is that it requires viruses that can kill cells. Not all viruses are able to cause cell death, meaning that the plaque assay cannot be used for all viruses.
